FIG. 35A is a top schematic view of a conventional elastic wave element. FIG. 35B is a sectional schematic view taken on line 35B-35B (in an extending direction of electrode fingers) of FIG. 35A. FIG. 35C is a graph showing acoustic velocity of a main elastic wave in FIG. 35B.
Elastic wave element 101 includes lithium tantalate piezoelectric substrate 102, IDT (Inter Digital Transducer) electrode 103, and reflector electrodes 104. IDT electrode 103 is formed on piezoelectric substrate 102 and excites a main elastic wave having wavelength λ. Reflector electrodes 104 are formed on piezoelectric substrate 102 in such a manner that they sandwich IDT electrode 103.
IDT electrode 103 includes first bus bar electrode 121 and second bus bar electrode 221 facing first bus bar electrode 121. Furthermore, IDT electrode 103 includes first electrode fingers 123 extending from first bus bar electrode 121 to second bus bar electrode 221, and second electrode fingers 223 extending from second bus bar electrode 221 to first bus bar electrode 121. Furthermore, IDT electrode 103 includes first dummy electrodes 122 extending from first bus bar electrode 121 to second bus bar electrode 221, and second dummy electrodes 222 extending from second bus bar electrode 221 to first bus bar electrode 121.
Bus bar electrode region 106, dummy electrode region 107, intermediate region 108, and alternately disposed region 109 are formed on piezoelectric substrate 102. Bus bar electrode region 106 includes first bus bar electrode 121 or second bus bar electrode 221. Dummy electrode region 107 includes first dummy electrodes 122 and first electrode fingers 123 or second dummy electrodes 222 and second electrode fingers 223. Alternately disposed region 109 includes first electrode fingers 123 and second electrode fingers 223 which are alternately disposed. Intermediate region 108 includes any one of first electrode fingers 123 and second electrode fingers 223.
Furthermore, by forming, for example, heavy metal layer 110 on a part of bus bar electrode region 106 so as to increase the thickness, a main elastic wave in alternately disposed region 109 can be trapped in elastic wave element 101.
However, in elastic wave element 101, heavy metal layer 110 can be formed only in bus bar electrode region 106 of IDT electrode 103. Therefore, an effect of trapping the main elastic wave in alternately disposed region 109 is not sufficient.
In a resonator formed of lithium tantalate piezoelectric substrate 102, energy of the main elastic wave excited by IDT electrode 103 tends to be shifted to a region in which acoustic velocity is high. This is because the lithium tantalate piezoelectric substrate has an anisotropy index of γ<0 in the propagation direction.
As shown in FIG. 35C, the acoustic velocity of the main elastic wave in intermediate region 108 is higher than the acoustic velocity of the main elastic wave in alternately disposed region 109. Therefore, the main elastic wave of alternately disposed region 109 leaks into intermediate region 108, so that characteristic loss of elastic wave element 101 may occur.
Note here that prior art literatures related to the present invention include Patent Literatures 1 and 2.